An Introductory Course of Quantitative Chemical Analysis eBook

Procedure.—­Weigh out into 500 cc.
beakers two portions of 0.175-0.200 gram each of pure
arsenious oxide. Dissolve each of these in 10
cc. of sodium hydroxide solution, with stirring.
Dilute the solutions to 150 cc. and add dilute hydrochloric
acid until the solutions contain a few drops in excess,
and finally add to each a concentrated solution of
5 grams of pure sodium bicarbonate (NaHCO_{3}) in water.
Cover the beakers before adding the bicarbonate, to
avoid loss. Add the starch solution and titrate
with the iodine to the appearance of the blue of the
iodo-starch, taking care not to pass the end-point
by more than a few drops (Note 1).

From the corrected volume of the iodine solution used
to oxidize the arsenious oxide, calculate its relation
to the normal. From the ratio between the solutions,
calculate the similar value for the thiosulphate solution.

[Note 1: Arsenious oxide dissolves more readily
in caustic alkali than in a bicarbonate solution,
but the presence of caustic alkali during the titration
is not admissible. It is therefore destroyed by
the addition of acid, and the solution is then made
neutral with the solution of bicarbonate, part of
which reacts with the acid, the excess remaining in
solution.

As the reaction between sodium thiosulphate and iodine
is not always free from secondary reactions in the
presence of even the weakly alkaline bicarbonate,
it is best to avoid the addition of any considerable
excess of iodine. Should the end-point be passed
by a few drops, the thiosulphate may be used to correct
it.]

DETERMINATION OF COPPER IN ORES

Copper ores vary widely in composition from the nearly
pure copper minerals, such as malachite and copper
sulphide, to very low grade materials which contain
such impurities as silica, lead, iron, silver, sulphur,
arsenic, and antimony. In nearly all varieties
there will be found a siliceous residue insoluble
in acids. The method here given, which is a modification
of that described by A.H. Low (!J. Am.
Chem. Soc.! (1902), 24, 1082), provides for the
extraction of the copper from commonly occurring ores,
and for the presence of their common impurities.
For practice analyses it is advisable to select an
ore of a fair degree of purity.

Procedure.—­ Weigh out two portions
of about 0.5 gram each of the ore (which should be
ground until no grit is detected) into 250 cc.
Erlenmeyer flasks or small beakers. Add 10 cc.
of concentrated nitric acid (sp. gr. 1.42) and heat
very gently until the ore is decomposed and the acid
evaporated nearly to dryness (Note 1). Add 5 cc.
of concentrated hydrochloric acid (sp. gr. 1.2) and
warm gently. Then add about 7 cc. of concentrated
sulphuric acid (sp. gr. 1.84) and evaporate over a
free flame until the sulphuric acid fumes freely (Note
2). It has then displaced nitric and hydrochloric
acid from their compounds.